Variable reactance and control circuit therefor



VARIABLE REACTANCE AND CONTROL CIRCUIT THEREFOR Filed Dec. 51, 1941 2 Sheets-Sheet 1.

Fdm, 6, 194-5, M g gsgy 2,36%,43

VARIABLE REACTANCE AND CONTROL CIRCUIT THEREFOR Filed Dec. 51, 1941 2 Sheets-Sheet 2 INVENTOR crystal sections.

Patented Feb. 6, 1945 VARIABLE REAOIANCE AND CONTROL CIRCUIT THEREFOR Murray G. Crosby. Riverhcad, N. Y., assiguor to Radio Corporation of America, a corporation of Delaware Application December 31, 1941, Serial No. 425,022

3 Claim.

This application concerns a variable condenser, which may be used for automatic frequency control and frequency modulation purposes, wherein a piezo-electric bending or twisting unit is the condenser actuating element. The device provides a simple and compact means of frequency modulating an oscillator or phase modulating a tuned circuit. It is especially suitable for automatic frequency controlling the high frequency oscillator of an U.-H.-F. receiver since it requires little or no power for operation and since the reactance tubes usually used for such purposes are relatively insensitive at the ultra high frequencies.

in describing my invention more in detail, reference will be made to the attached drawings wherein,

Figs. la and lb illustrate a piece-electric cryss tal unit arranged in such a manner as to bend when excited by variable potentials as shown.

In Fig. 1c is shown, rather schematically, a crystal unit of the type illustrated in Figs. 1c and lb, cooperatin with elements of a reactance such as a condenser to provide a variable reactive effect.

Fig. id illustrates a modification of the ar rangement of Fig. 10.

Figs, 2 and 3 illustrate high frequency circuits associated with the variable reactance of the prior figures. In Fig. 2 the variable reactance is used to control in a novel manner an oscillator in a receiver while in Fig. 3 the variable reactance is shown connected in a novel manner with the generator of a transmitter to control the frequency thereof.

Fig. la shows a section of the type of crystal bending unit used here to provide variable capacity. Crystals 6 and 8 are cemented or otherwise bound together and associated with plates iii, i2, and it, of which plate 14 is between the The crystal axes are so oriented that when voltage is applied to terminals l and i2 and thence differentially to the crystal se ments, one of the crystals is compressed in its lengthwise dimension and the other is elongated. This causes the unit to operate like a bi-metallic thermal element so that when voltages are applied to the terminals it and it the unit bends as shown in Fig. lb.

Composite crystal. units of the nature involved here, in which Rochelle salt crystals are used, are described in an article by C. Baldwin Sawyer, entitled The use of rochelle salt crystals for electrical reproducers and microphones, pub- 'lished in the Proceedings of the I. R. E. for November, 1931.

Figs. 1c and 1d show methods of utilizing this bending movement of the crystal unit to form a variable capacity which varies in proportion to the voltage applied to the crystal electrodes. In Fig. 1c the crystal 20, which may be made up of two sections arranged and excited as described above in connection with Fig. la, is clamped to holder 18 by means of bar 22 and screw 24. The bottom of electrode IS on the crystal acts as one plate of the condenser and plate 28 act as the other plate. A dielectric 28, which may consist of mica or a similar material, may be placed be; tween the condenser plates if desired. The clamping oi the crystal unit is adjusted so as to leave an air gap at the free end of the crystal for the normal position of the condenser without voltage applied. Then, as voltage is applied by leads to and 32 to the crystal electrodes, the bending movement of the crystal either increases or decreases the spacing between the condenser plates i9 and 26 and thereby decreases or increases the capacity. The variable capacity may be used by connections 34 and 36 of which 36 is grounded as is plate 119.

Fig. 1d shows an arrangement in which the crystal bending unit operates a compression type condenser. The condenser consists of a fiat plate Mi, a dielectric such as mica t2, and a curved plate it. The curved plate without pressure allows an air gap to exist between the plate and the dielectric. When voltage is applied to tenninals t6 and iii to cause crystal unit Ell to bend.

the end of the bending unit presses down on the curved condenser plate M through an insulating material such as rubber 52. By means of the two clamps 54 and 58, with their screws 58 and $0 cooperating with support ti on holder 59, the free end of the crystal may be moved up and down to adjust the normal position of the condenser. Application of voltage to terminals t6 and t8 then causes the bending unit to vary the degree of compression in the compression type condenser and consequently vary the capacity. (Jonncctlon to the condenser plates is afforded by terminals 62 and 6 5.

The controllable reactance described above may be put to wide use in the radio art. For example, the controllable reactance may be used to tune an alternating current circuit of any type such as, for example, a circuit wherein alternating current to be phase modulated flows, or an oscillation generator circuit the frequency of which is to be modulated by the control potentials ap- "any type of receiver illustrated diagrammatically by box 18. This receiver includes in 16 a mixer tube supplied by oscillations from genorator tube It and anintermediate frequency amplifier feeding tube ll. Normally, tuned circult 12! might be tuned above the mean intermediate frequency and tuned circuit 14 below the mean frequency while the primary tuned circuit 10 might be tuned to approximately the mean frequency. The tuned circuits are damped by resistors l8, l1, and Is to cause the discrim1- nator to have the proper band width. Detectors II and 82 detect the amplitude modulations from the discriminator so that modulation output, where I8 is a frequency modulation receiver, and

automatic frequency control potentials appear across resistors 84 and 88. The automatic frequency control potentials are fed through resistor l8 and chokes 80 and 92 to the electrodes of crystal element 20. Resistance 88 and condenser ll form a time constant circuit which removes the fast variations and allows the slow variations characteristic of slow changes in the mean frequency of the received wave energy to pass to the piezo-electric bending unit. The type of variable condenser shown here is similar to the one of Fig. 10. Crystal electrodes or plates 20 and II are connected to the chokes 90 and 92. Plate acts as one plate of the condenser and the bottom electrode l9 of the crystal acts as the other plate. The crystal is clamped by means of-bars 22 and II as shown in Fig. 1c. 9! is an insulating separator. Connection is made to the tuned circuit 84 of oscillator 13 through condenser II and lead 9!. Voltage from this oscillator is fed to the mixer tube via lead I00. This oscillator may be the high frequency oscillator of a superheterodyne or the second oscillator of a double intermediate frequency superheterodyne. Chokes l0 and 92, in connection with condenser I and resistance l8, act as a filter to isolate the oscillator circuit 94 from the detector circuit l0, 12, etc.

Fig. 3 shows an application of the piezo-elec-. trio variable condenser to a frequency modulator for the production of frequency modulated waves. In Fig. 3, tube I oscillates because tuned circuit I, I0! couples its grid I01, cathode I09 and anode III in a regenerative circuit of the Hartley type. The plaza-electric variable condenser H0 forms a part of the variable capacity of the tuned circuit since its one electrode H8 is connected to one terminal of inductance Hi8 and its other electrode H9 is connected to ground. Modulation input is applied to jack H2 through isolating filter H4, H6 to operate the variable condenser. This filter prevents high frequency energy from reaching the modulation circuits. Output from the oscillator circuit is led to unit H8 which contains frequency multipliers, limiters, and power amplifiers. The output of unit i I8 may be applied to an antenna or other transmission medium.

While I have shown my novel variable reactance in an oscillation generating circuit for control and modulation purposes it will be understood that the same may be used in a tuned circuit of any type such as, for example, one in which wave energy of carrier wave frequency to be modulated in phase flows.

It will be understood that the torque units described in the above mentioned article by C. B. Sawyer may be used as the actuating element in a manner similar to the use of the bending units.

What is claimed is:

1. A reactancc including, a composite piezoelectric crystal comprising adjacent sections the axes of which are at angles with respect to each other, whereby the sections change in form in different directions when the composite crystal is excited to produce resultant movement, and a reactance element mechanically connected to said crystal.

2. A capacitive reactance including, a composite piezo-electric crystal comprising adjacent sections the axes of which are at angles with respect to each other, whereby the sections change in form in different directions when the composite crystal is excited to provide resultant movement, and a condenser element mechanically linked to said crystal.

3. In a wave length modulation system. an oscillation generator including an electron discharge device having electrodes connected in oscillation generating circuits, a composite piezoelectric crystal comprising two sections the axes of which are at angles with respect to each other. whereby the sections change in form in different directions when the composite crystal is excited, a reactive element mechanically connected to said composite crystal and included in said oscillation generating circuit to determine in part the frequency of the oscillations generated, and means for applying a control potential to points on said crystal to cause relative movement. of the sections thereof and thereby movement of said reactance element.

MURRAY G. CROSBY. 

